1. Field of the Invention
This invention relates to a new acid-resistant copper chromite spinel catalyst for the direct hydrogenation of fatty acids to fatty alcohols.
2. Statement of Related Art
Fatty alcohols, i.e. predominantly linear, monofunctional alcohols having chain lengths of 8 or more carbon atoms, and their production are described in detail in the literature, for example, in Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Vol. II, pages 427 to 445. Preferred starting materials for their production are the fatty acids and fatty acid mixtures occurring in natural fats and/or oils which may be converted into fatty alcohols of corresponding chain length by catalytic hydrogenation. Through the use of the fatty acids to be reduced in the form of their methyl esters, the catalysts in particular are protected against aggressive attack by the free carboxyl group, so that industrial processes can be operated for sufficiently long periods with satisfactory volume-time yields. Today, therefore, the predominant quantity of native fatty alcohols is produced from fatty acid methyl esters by a process in which the distilled methyl esters are passed in the liquid state, together with a large excess of hydrogen, over fixedly arranged copper-containing mixed oxide catalysts, such as for example copper/zinc catalysts, at temperatures above 200.degree. C. and under pressures of from about 250 to 300 bar.
The copper-mixed oxide catalysts obtained by coprecipitation via the wet route are used as particulate catalysts or extrudates and, before use, are generally reduced in the plant or installation. They are not acid-resistant. Accordingly, it has not hitherto been possible in practice to produce native fatty alcohols by direct hydrogenation of the free fatty acids.
It is known from Ullman, loc. cit., that the hydrogenation of free fatty acids to fatty alcohols can be carried out by the suspension process using copper (II) chromite catalysts. However, this method can only be effectively used when the copper (II) chromite catalyst is obtained by decomposition of the copper ammonium chromate complex initially obtained and subsequent washing with acetic acid. Catalysts prepared in this way are particularly expensive and, in practice, can only be used for suspension hydrogenation. Acid-washed copper (II) chromite can only be tabletted with considerable difficulty, if at all, and accordingly cannot be converted into abrasion-resistant or mechanically strong extrudates or other shapes. Any attempt to achieve this increase in strength by after-annealing weakens the effect of the catalyst. Attempts to apply acid-washed copper (II) chromite to catalyst supports, such as silica gel or aluminium oxide for example also produces technically unuseable catalysts. The support is attacked and the catalyst is readily washed away.
According to the relevant patent literature, fatty acid esters, more especially fatty acid methyl ester, and free fatty acids are therefore simultaneously used as starting materials for the hydrogenation reaction to saturated and/or unsaturated fatty alcohols (cf. for example U.S. Pat. No. 3,193,586; U.S. Pat. No. 3,173,959; German Pat. Nos. 2,513,377 and 2,613,226). So far as industrial application is concerned the above processes have to be evaluated entirely differently according to whether the fatty acid esters of the free fatty acids are used as starting material for hydrogenation. It is generally known that the significant advantages of fixed-bed catalysis using solid catalysts do not apply to the processing of a starting material consisting of or containing free fatty acids. The corrosive effect of the free fatty acids at high temperatures and pressures on the solid catalysts which, basically, have been successfully used in the reduction of methyl esters is so great that it has not hitherto been possible to consider any of the above proposals for practical application in the reduction of free acids.
In practice, therefore, the situation is remedied by initially introducing relatively large quantities of fatty alcohol into the hydrogenation reactor and subsequently adding free fatty acid under hydrogenation conditions. However, this process requires relatively large reactors. It only achieves conversions of 96% whereas processes using fixed-bed catalysts achieve conversions of 99% and higher.
As discussed above, copper chromite catalysts are highly active catalysts for the hydrogenation of fatty acid esters and triglycerides of oils and fats. The direct hydrogenation of fatty acids with catalysts of this type has not hitherto been possible because copper (II) chromite (CuCr.sub.2 O.sub.4) is not acid-resistant, i.e. is dissolved by the action of the free fatty acid. This applies in particular to the CuO present in the catalyst.